If you know what plugins are causing the issue, you can use the Jenkins CLI to remove or downgrade those plugins. For instance:
java -jar jenkins-cli.jar -s http://localhost:8080/ install-plugin SOURCE ... [-deploy] [-name VAL] [-restart]
Installs a plugin either from a file, an URL, or from update center.
SOURCE : If this points to a local file, that file will be installed. If
this is an URL, Jenkins downloads the URL and installs that as a
plugin.Otherwise the name is assumed to be the short name of the
plugin in the existing update center (like "findbugs"),and the
plugin will be installed from the update center.
-deploy : Deploy plugins right away without postponing them until the reboot.
-name VAL : If specified, the plugin will be installed as this short name
(whereas normally the name is inferred from the source name
-restart : Restart Jenkins upon successful installation.
I believe you can also just drop an .hpi file into a plugins directory in your Jenkins installation, but I've never needed to resort to this solution before, so I can't say how well it works.